1. Field of the Invention
The invention relates to an on-board lighting device in a motor vehicle and a related method for controlling a light source.
2. Description of the Related Art
Nowadays, motor vehicle door handles are fitted with devices for detecting the presence of a user, i.e. the approach and/or contact of the user's hand. Detection of the presence of a user coupled with recognition of an electronic “hands-free” badge for remotely controlling access to the vehicle that is carried by this user enables the doors of the vehicle to be locked and unlocked remotely. Thus, when the user carrying the corresponding electronic badge that has been identified by the vehicle approaches the handle or touches the door handle of his vehicle, the doors of the vehicle are automatically unlocked. By approaching or pressing a specific point on the door handle of the vehicle, referred to as the “unlocking zone”, the door opens without having to unlock it manually. Conversely, when the user, still carrying the necessary badge identified by the vehicle, wishes to lock his vehicle, he closes the door of his vehicle and approaches or momentarily presses another specific point of the handle, referred to as the “locking zone”. This gesture enables the doors of the vehicle to be locked automatically.
Presence detection devices usually include two capacitive sensors, in the form of two electrodes linked electrically to a printed circuit and built into the handle, each one in a specific locking or unlocking zone. Generally, one electrode is dedicated to each zone, i.e. one electrode is dedicated to detecting the approach and/or contact of the user's hand on the locking zone and one electrode is dedicated to detecting the approach and/or contact of the user's hand on the unlocking zone. These detection devices work as follows:                one electrode, when energized, emits an electrical field that defines a detection zone (locking or unlocking zone),        the approach of the user's hand to this detection zone disturbs this electrical field and modifies the capacitance at the terminals of said electrode,        measuring the variation in this capacitance then makes it possible to detect the approach of the user's hand in said zone, in this case towards the handle,        once this detection has been performed, the capacitive sensor then sends an unlocking/locking instruction to the unlocking/locking system of the door.        
These presence and/or approach detection devices are usually built into a module, located in the door handle and also comprising a radio frequency antenna, used to recognize the “hands-free” access badge carried by the user, as well as one or more light sources illuminating certain specific zones of the vehicle or of the handle. It is common for each door handle of the vehicle to be fitted with such a light source.
These light sources are usually, on account of their compactness and resistance to vibrations, light-emitting diodes (LEDs), preferably white in color. Depending on the applications, these white light-emitting diodes illuminate the locking/unlocking zones of the handle in which they are located, in order to guide the user towards these zones:                if an authorized hands-free access badge has been recognized near to the vehicle, or        alternatively to confirm to the user that his presence has been detected, once he has moved his hand close to these locking/unlocking zones.        
Illuminating and extinguishing these diodes is controlled by a body controller module (BCM) on board the vehicle. The body controller module is an electronic processor that receives data from different sensors located on the vehicle, in this case receiving user presence detection information from the presence detection sensors located in the door handles of the vehicle. As a function of this information, for example, the body controller module decides to illuminate and/or extinguish the light-emitting diode located in the handle near to which the user's presence has been detected. The body controller module, the presence detection sensors and the light-emitting diodes form a lighting device carried on board the vehicle.
However, a major problem with diodes is that, even when they come from the same production line, they have different luminous intensities (expressed in millicandelas, mcd). This variability in luminous intensity comes from the method used to manufacture the light-emitting diodes themselves. Luminous intensity can vary from, for example, 1500 mcd, to nearly double that, for example 2800 mcd, for two light-emitting diodes from the same production line.
This variation in luminous intensity is such that a user can see it on a vehicle fitted with several diodes. For example, if the vehicle is fitted with light-emitting diodes on each door handle, since it is particularly easy to compare luminous intensity between two handles located on the same side of the vehicle, if the difference in luminous intensity between these two light-emitting diodes is visible, the user of the vehicle is liable to think that the dimmer diode is faulty. However, this is not the case.
One solution to this problem is to select the light-emitting diodes at the end of the production line, sort them into classes according to the luminous intensity thereof, then fit the vehicle with diodes all taken from the same class. However, this solution is very expensive in terms of time and the logistics are complicated.
Another solution involves fitting the vehicle with light-emitting diodes from different classes, then paring them on the vehicle, adding a resistor to each diode as a function of the luminous intensity class thereof. The resistor modifies the intensity of the charge flowing through the light-emitting diode, enabling the luminous intensity to be corrected by imposing a fixed value thereon, that is the same for all of the light-emitting diodes fitted to a given vehicle. In this case, the simplest solution available is to select resistors that make it possible to obtain the luminous intensity of the class having the lowest intensity of all of the diodes.
FIG. 1 shows a lighting device D in the prior art carried on board a motor vehicle (not shown).
The lighting device D includes:                a body controller module 10, connected electrically:                    firstly to four approach and/or contact detection sensors 100, 200, 300, 400 arranged in parallel, in order to supply them with a current (VBAT) and connect them to ground (GND),            and secondly to four light-emitting diodes D1, D2, D3, D4 arranged in parallel, each being connected in series to a regulating resistor R1, R2, R3, R4, itself connected to ground.                        
Each set comprising an approach and/or contact detection sensor 100, 200, 300, 400, a diode D1, D2, D3, D4 and a related resistor R1, R2, R3, R4 is built into a vehicle door handle P1, P2, P3, P4.
The body controller module 10 powers the approach and/or contact detection sensors 100, 200, 300, 400 and receives information I1, I2, I3, I4 as feedback from these latter relating to detection of the approach and/or contact of a user near to the locking/unlocking zones. On receipt of this information I1, I2, I3, I4, the body controller module 10 then sends a lighting signal using pulse-width modulation (PWM) to the four diodes D1, D2, D3, D4 located in the door handles P1, P2, P3, P4 of the vehicle in order to illuminate or extinguish them, as applicable. This PWM lighting signal is shown in FIG. 3. It involves a succession of changes from a low state DB (0%) to a high state DH (100%), and changes from a high state (100%) to a low state (0%), also known as pulses Imp (see FIG. 3). Such signals usually have three phases:                a progressive illumination phase A, to progressively illuminate the diode D1, D2, D3, D4, during which the durations of the high state, DH increase progressively,        a continuous illumination phase B of the diode D1, D2, D3, D4, during which the PWM signal is constantly in the high state DH, and        a progressive extinction phase C, during which the durations of the high state DH are progressively reduced, in order to progressively extinguish the diode D1, D2, D3, D4.        
As with all pulse-width modulation (PWM) signals, the ratio between the durations of the high states DH and the durations of the low states DB determines a duty cycle:
      R    ⁢                  ⁢    c    =            D      ⁢                          ⁢      H              D      ⁢                          ⁢      B      
The intensity of this lighting signal PWM is modified differently through each diode D1, D2, D3, D4 through the presence of the resistors R1, R2, R3, R4. The value of the resistors R1, R2, R3, R4 is selected as a function of the luminous intensity class of each diode D1, D2, D3, D4, in order to obtain four diodes having substantially equal luminous intensity.
This solution has several drawbacks:                this solution is complicated, since it requires the luminous intensity class of each diode D1, D2, D3, D4 fitted to the vehicle to be known, and a dedicated resistor R1, R2, R3, R4 needs to be physically paired with each diode, by means of an additional manual operation on the production line,        the logistics are difficult because the number of diodes produced in each class needs to be forecast in order to order an equal number of corresponding resistors,        sorting at the end of the production line is costly. In order to reduce the cost of this sorting, it is essential to reduce the number of classes, for example to four or five classes. Consequently, this means only four or five resistors paired with these classes. This reduced number of resistors is insufficient to cover the variation in luminous intensity that still exists within a single class. Therefore, there remains a risk of differences in the luminous intensity of the diodes located on the vehicle, which remain visible to the user.        